1. Field of the Invention
The present invention is in the field of ceramic capacitors, and more particularly pertains to a test for rapidly determining whether a ceramic capacitor of a given batch of the type more particularly hereinafter set forth will evince satisfactory IR (Insulation resistance) characteristics.
2. The Prior Art
There is shown and described in U.S. Pat. No. 4,081,857, by way of example, a series of processing steps and formulations for the fabrication of ceramics to be utilized as the dielectric element of ceramic capacitors. The patent is representative of typical formulations and methods of fabrication of a fired ceramic which employ Barium titanate as a principal ingredient in conjunction with a bismuth compound: Bismuth-containing ceramic dielectric materials, in addition to Barium titanate include stannates, zirconates, niobates, titanates, oxides and solid solutions, and unfired mixtures of the foregoing are known to have certain highly desirable characteristics including, without limitation, a high capacitance per unit size. In particular this patent pertains to formulations which are also referred to as grain growth inhibited barium bismuth titanate ceramics used for BX,X7R or NPO applications.
A desirable and, in most instances, a necessary characteristic of capacitors of the type described, is a high IR characteristic, particularly at high temperatures. More specifically, it is considered highly desirable for all such capacitors to evince an initial IR characteristic of 300 or greater measured in megohms/microfarad (M.OMEGA...mu.f) using the rated voltage at 125.degree. C.
Initial IR characteristics of from 100 to 300 M.OMEGA...mu.f are considered to be partially acceptable, whereas capacitors having an initial IR characteristic of lower than 100 M.OMEGA./.mu.f are considered to be useful only in limited applications.
Typical IR testing is done on a per batch basis--that is to say, a batch of capacitors is fabricated and representative samples thereof are selected for testing. It is generally recognized that the entirety of a given batch will evince the same IR characteristics as the selected samples.
The factors which can affect the IR characteristic are only partially understood and minute changes in purity of materials, times and temperatures of processing, etc. are apparently critical to the IR characteristic. Notwithstanding great care taken in the selection of starting materials, compounding of formulae and processing, it is apparently impossible consistently to assure the fabrication of capacitors of the type described having IR characteristics within an acceptable or desirable range.
Current IR testing techniques are time-consuming and add greatly to the expense of producing ceramic capacitors. Typically, the completion of a testing program of a given batch will require a time lapse of some six weeks.
In accordance with current techniques, the selected samples of a given batch are terminated in conventional manner and the terminations fitted with leads. The termination and lead attachment procedures normally take approximately two days in the laboratory. The samples are then measured for initial IR at 25.degree. C. using the rated voltage. The IR measurement is carried out at the temperature of interest by applying the rated voltage to the capacitor in series with a current limiting resistor. The current is read by using an electrometer (for example, but not limited to, Model 26200 electrometer from Keithley Instruments Inc., Cleveland, Ohio) and the IR is calculated using the Ohm's law.
Assuming that the initial IR characteristics at 25.degree. C. are satisfactory the units are placed in a chamber at 125.degree. C. and the initial IR at this temperature is determined according to the procedure described above. The samples are then placed in a chamber and subjected to twice the rated voltage at 125.degree. C. for various time intervals till 1,000 hours are completed. The samples are removed at each of these pre-determined time intervals during the above voltage treatment and the IR at 125.degree. C. is evaluated to study the degradation, if any, of IR due to this treatment.
It is apparent from the above procedure, also known as life test, that the further processing of the entire batch of fired chip capacitors must be set aside for a protracted period as a result of current six week long IR test techniques.